1. Field of the Invention
The present invention relates to a table cloth matrix of EPROM memory cells with an asymmetrical fin.
2. Description of Related Art Including Information Disclosed Under 37 CFR.sctn..sctn.1.97-1.99
In the field of memories with EPROM cells one of the fundamental problems is the density of the cells; it is important to succeed in compacting the memory cells as much as possible the better to exploit the available space and to improve performance. For this reason research is oriented towards the study of new structures which overcome these problems.
One of these new structures is the so-called "table cloth" matrix, wherein two groups of source lines and one group of drain lines parallel and alternated to one another are intersected perpendicularly by control gate lines parallel to one another, while floating gate areas are interposed between said source and drain lines beneath said control gate lines.
This configuration is described in the Italian patent application No. 23479 A/84 dated Nov. 7, 1984 and corresponding U.S. Pat. No. 4,792,925 assigned to the assignee of the present application and allows the attainment of good levels of miniaturization and of cell density.
One of the main defects of this structure is the low capacitative coupling between the control gate and the floating gate of each cell.
In fact the coupling area between the gates is practically identical to that between the floating gate and the substrate so that the capacitative ratio is lower than that of standard EPROM cells, and the cell's writing characteristics are thus worsened. The memory is thus highly sensitive to the so-called "drain turn-on", a parasitic effect which can lead to softerasing effects in non-selected cells.
An improvement on this structure is illustrated in the Italian patent application No. 21560 A/88 dated Jul. 29, 1988, and corresponding European Application No. 0352830 assigned to the assignee of the present application. In this structure islands of field oxide are provided in the matrix zone, which, while allowing an improvement in the capacitative coupling between the gates, involve a greater complexity of accomplishment and a lower degree of obtainable integration.
Lastly, in the Italian patent application No. 22848 A/88 dated Dec. 5, 1988and corresponding U.S. Pat. application No. 444,796 filed Dec. 1, 1989 assigned to the assignee of the present application, a variant of the table cloth matrix structure is described. In this Application there are provided field oxide areas formed between the control gate lines, and the floating and control gates are provided with symmetrical lateral fins which are superimposed over the field oxide areas. In this structure the control gate is in contact with the underlying stratum only at narrow lateral fins, while in the central zone contact is prevented by an oxide layer residue from the planarization stage included in the manufacturing process of the matrix. Thus the contact area between the gates is extremely small, and this worsens contact, if it does not altogether prevent it.